This invention relates to a screw utilized in an injection moulding machine and an extrusion moulding machine for plasticizing resinous materials.
In a screw for moulding resinous materials in cooperation with a barrel at high temperatures in which the screw is rotated to advance the helical flight while working the resinous meterial initially in the form of a solid bed into a melt, it is necessary to uniformly heat the plasticized and molten resinous material and to thoroughly knead and homogeneously melt the same. It is also necessary to thoroughly degass the molten resinous material. The molten resinous material must have short and uniform thermal hysteresis.
A conventional screw can not always fulfill these requirements. More particularly, in an ordinary screw, not yet molten resinous material and molten material are separated in the groove of the screw. Such tendency is especially remarkable in a crystalline resin. Where a so-called solid bed, that is, a not yet molten solid phase is formed and the solid bed is sequentially melted starting from the contact surface between the solid phase and the inner wall of the barrel, mutual heat exchange between the solid bed and the molten resin and admixture thereof are not sufficient. Thermal hysteresis also differs between the resinous material which has melted at the early stage of the screw moulding and that melted near the last or exit stage of the screw moulding. This also prevents homogenization of the molten resinous material. To exclusively ensure homogenization, according to a prior art design, the length of the screw was made sufficiently long. This design not only degrades the thermal hysteresis but also increases the cost of manufacturing. Thus, a number of solutions have hitherto been proposed to ensure homogeneous melting. For example, it has been proposed to implant pins or keys in the grooves of the screw, to remove flight for several pitches and substitute barriers or to make wavy the flight. However, these designs are not yet satisfactory.
In a copending patent application Ser. No. 757,042 filed on Jan. 5, 1977, assigned to the assignee of the present application, there is disclosed a screw provided with helical flight and valley wherein the bottom surface of valley is divided into a plurality of parallel strip shaped portions having wavy contours with a predetermined phase difference therebetween thereby forming a plurality of cross points between the wavy contours along the dividing plane. Although this construction can efficiently rupture solid phase resin to homogenize the molten resin and to improve heat distribution and blending, the valleys of the divided portions communicate with each other at the cross point thus decreasing the thrust pressure.